vineet As 1.8 does not support PHP 7.2, your only option is to upgrade to osTicket 1.12. It's incredibly easy, but you'll have to redo your customisations. Hopefully they've all been recorded!
EDIT: I'm assuming by customisations you mean you've changed the core files? If it's just templates and other changes made via osTicket admin panel, these will migrate along with the upgrade.